A plurality of systems uses a battery implemented as a battery pack or battery array, including a plurality of battery cells connected in series with each other.
When such a battery cell is charged to a much higher voltage or a much lower voltage than the voltage within a rated charge range, it may be dangerous.
Imbalance in the charged state of battery cells is caused by various factors, and occurs during the manufacture of batteries or the charge/discharge of batteries. In the case of lithium ion cells, the manufacture of cells is strictly controlled within a company to minimize the differences between the capacities of the cells of a battery array. However, imbalance or inequality between cells may occur due to various factors, regardless of the states of the cells, in which balance or equality is maintained after the cells are initially manufactured.
The factors influencing the imbalance of cells may include, for example, the chemical reactions, impedances and self-discharge rates of respective cells, reduction of the capacities of the cells, variation in the operating temperatures of the cells, and other types of variation between the cells.
Inconsistency in the temperature of cells is an important factor responsible for causing imbalance in cells. For example, “self-discharge” is caused in a battery cell, and is a function of a battery temperature. A battery having a high temperature typically has a self-discharge rate higher than that of a battery having a low temperature. As a result, the battery having a high temperature exhibits a lower charged state than the battery having a low temperature, with the passage of time.
Imbalance is a very series problem in the charged state of a battery. For example, this problem may typically occur in electric vehicles, and the capability of a battery to supply energy is limited by the battery cell having the lowest charged state.
If one of series-connected batteries is fully consumed, other battery cells lose the ability to continue to supply energy. This is the same even if the other battery cells of the battery still have the ability to supply power. Therefore, an imbalance in the charged state of battery cells reduces the power supply capability of the battery.
Of course, the above description does not mean that when one or more battery cells are consumed the supply of power by the remaining battery cells is completely impossible. However, it means that, only in the case of series connection, even if one or more battery cells are fully consumed, the battery can be continuously used as long as charge remains in the remaining battery cells, but, in that case, voltage having a reversed polarity is generated in the battery cell which has been fully discharged, and, as a result, the battery cell may be in danger of explosion due to the overheating thereof, or due to the generation of gas, and thus the battery loses power supply capability.
Various methods of correcting an imbalance in the charged state of battery cells have been proposed, and one of the methods is shown in FIG. 1.
FIG. 1 is a diagram showing the construction of a conventional charge equalization apparatus with series-connected battery cells using current switching.
Referring to FIG. 1, a plurality of battery cells B1 to BN is connected in series, and is connected in parallel with the primary windings M11 to M1N of a transformer T. Further, switches S1 to SN are connected in series with respective primary windings M11 to M1N, and the battery cells B1 to BN, connected in series, are connected in parallel with the secondary winding M2 of the transformer T. In this case, all of the primary windings M11 to M1N are wound around a single common core.
The primary windings M11 to M1N of the transformer T have the same number of turns and the same polarity, and the secondary winding M2 of the transformer T has polarity opposite that of the primary windings M11 to M1N, and is connected in series with a rectifier diode D. The cathode of the rectifier diode D is connected to the anodes of the series-connected batteries B1 to BN, so that current flowing from the cathode of the rectifier diode D flows into the anodes of the series-connected battery cells B1 to BN.
Further, a voltage sensing and switch drive signal generation unit 100 senses voltages at both ends of respective battery cells B1 to BN, and turns on/off the switches S1 to SN according to a preset scheme.
In detail, the voltage sensing and switch drive signal generation unit 100 senses the voltages of the battery cells B1 to BN, and drives a corresponding switch S1 to SN when the voltage of a specific one of the battery cells B1 to BN is higher than a predetermined voltage, thus discharging the charge from the specific one of the battery cells B1 to BN. In this case, the discharged charge flows through the transformer T, with the charge converted into magnetic energy by the transformer T, the magnetic energy is converted back into charge when it encounters a battery cell B1 to BN having a relatively low potential, and the charge flows into the battery cell B1 to BN. At this time, a larger amount of charge flows into a battery cell B1 to BN having a relatively lower potential, thus realizing charge equalization.
However, according to the prior art, there is a problem in that, since a plurality of primary windings corresponding to the number of battery cells must be wound around a single common core, it is difficult to manufacture a transformer when the number of series-connected battery cells increases.
Further, in the prior art, the voltages of the series-connected battery cells are applied to the primary windings for a period during which a switch is turned off to prevent the saturation of the transformer. At this time, the turns ratio of the primary winding to the secondary winding is N (the number of series-connected battery cells), and thus it is difficult to manufacture a secondary winding as the number of battery cells increases.